A first desired feature of optical elements for daily use, such as sunglass lenses, is the reduction of glare, which term refers to the presence of areas or “hot spots” in the field of vision which are of sufficient brightness to cause visual impediment, such as temporary blurring of vision, or ocular fatigue. Glare often occurs when a patch of bright light is reflected from smooth, shiny surfaces, such as water, snow, roadway or glass, into the eye.
A second desired feature of optical elements for daily use, such as sunglass lenses, is the accomplishment of a proper reduction of the visual discomfort which may arise when an object is focused alternately and repeatedly in the presence of an illumino-environmental contrast now limited now high, such as for instance the contrast which exists, respectively, between an object and the ground or an object and the sky that has in itself a markedly higher luminance.
In this regard, some experimental tests carried out by the Applicant in sunny conditions in a varied environment in the presence of vegetation and at the latitude of about 43° North, have shown that the luminance of the sky may be from 1.5 to about 9 times higher than the luminance of the ground.
In such circumstances, the eye must vary continuously the pupillary diameter, consequently adjusting the exposure of the retina to the external light, to face alternately high and low luminosity conditions.
In spite of the efforts of the eye to adapt itself to an average situation, which in any case would not allow an optimal vision, this brings about a visual stress that may lead to visual straining and in some cases also to a partial (even though reversible) damage of the receptors located on the retina.
Such unfavorable phenomenon is always present and particularly marked when playing some sports activities in which the athlete or the sportsman repeatedly passes from the observation of objects in a relatively little luminous field to the observation of objects that are in a markedly more luminous field.
In some circumstances, the visual discomfort to the eye may even be enhanced to levels which impair a good vision and cause great visual stress when an illumino-environmental contrast is coupled to the presence of glare or “hot spots”. As can be easily understood, the stress generated to the eye is at a maximum level because the variation of pupillary diameter is forced not only when objects are alternately focused at different relative positions along a vertical direction, but also when they are alternatively focused against the ground in areas having different levels of glare along a horizontal direction.
Additionally, if a condition of relatively low diffused luminance but uniform glare is present at ground level, the eye will adapt itself to this illumination condition by lowering its sensitivity to take account of the glare with a consequent undesired loss of visual acuity.
An example of activity subjected to high visual stress due to the possible simultaneous presence of glare and of a marked illumino-environmental contrast, is driving a vehicle in bright sun conditions and alternately focusing the roadway and the instrumentation inside the vehicle.
Other examples of activities subjected to high visual stresses due to the particular illumination conditions described above are climbing in slopes with snow or climbing glaciers (the climber has continuously to look alternatively to the high glare ground and towards the summit with the sky as a background), or sailing (the skipper has the need of continuously looking alternatively to the sea surface or to objects having the sea surface as background and to the top of the sails with the sky as background).